Plasma reactors used in etch processing of semiconductor wafers in the manufacture of microelectronic integrated circuits tend to be limited by at least one of two factors: (a) etch selectivity and (b) etch rate. Capacitively coupled plasma reactors tend to exhibit superior etch selectivity but with only limited etch rate, while inductively coupled plasma reactors tends 2-D to exhibit superior etch rate but inferior etch selectivity. As semiconductor device geometries become increasingly smaller in the constant quest for higher on-chip clock speeds, these limitations become more critical. With smaller device geometries, the etching of contacts openings, for example, through multiple thin film layers, involves higher aspect ratios (deeper holes with smaller diameters).
Etch processes typically employ a fluorocarbon gas that dissociates into various species in the plasma. Dissociation of process gas molecules occurs because the RF power applicator (e.g., a capacitively coupled electrode pair or an inductively coupled coil antenna) produces xe2x80x9chotxe2x80x9d electrons in the plasma with kinetic energies sufficient to dissociate various molecular species into simpler species upon collision. The energy of the xe2x80x9chotxe2x80x9d electrons produced by the RF power applied to the plasma is distributed over some range, and different electron energies tend to produce different dissociated process gas species. Some species are polymer precursors that form an etch-resistant polymer coating on the non-oxygen-containing thin film surface. This feature improves etch selectivity when etching oxygen-containing materials such as silicon dioxide. Other species are etchants consisting of various fluoro-carbon species. The more volatile etchants are the more simpler chemical species, the most simple (free fluorine) being the most volatile etchant and the least selective. Greater dissociation produces simpler and more volatile etchant species and therefore leads to less etch selectivity. A process with less dissociation provides more complex etch species and therefore greater etch selectivity. Selectivity is also enhanced by the presence of more polymer precursor species. However, as the plasma becomes more polymer-rich, it is more prone to permit accumulation of polymer near the bottom of narrow deep openings, leading to etch stopping.
The smaller device geometries are more prone to etch stopping, in which the etch process stops before the hole has been etched to the required depth. In order to avoid such a problem, the process can be performed at higher plasma density (by increasing the plasma source power), but such changes can lead to greater dissociation within the plasma and a consequent loss of etch selectivity. Without increasing plasma density, the etch rate can be unacceptably low and etch stopping can occur.
As a result, the process window in which an acceptable balance between etch rate, etch selectivity and etch stopping can be realized is becoming smaller, particularly as semiconductor device geometries shrink. Currently, it is felt that a capacitively coupled reactor provides a better way around such problems because its superior etch selectivity permits the etch process to be carried out to completion with less damage to photoresist films and device structures, in comparison with an inductively coupled reactor. However, a capacitively coupled reactor typically cannot attain the higher etch rate of an inductively coupled reactor.
An inductively coupled reactor is disclosed, for example, in U.S. patent application Ser. No. 09/039,216 filed Mar. 14, 1998 by Brian Pu, et al. entitled, xe2x80x9cDistributed Inductively-Coupled Plasma Sourcexe2x80x9d and assigned to the present assignee. In the referenced application, the inductive antenna consists of a number of widely separated RF-driven coils in which successive coils have opposite polarity. The purpose of the opposing polarity coils is to reduce eddy currents formed in a conductive ceiling below the coil antenna so that RF power may be inductively coupled through the ceiling.
Accordingly there has seemed to be no solution to the problem of limited etch selectivity and limited etch rate. It would be highly advantageous if a plasma reactor could be provided having the etch rate of an inductively coupled reactor together with the etch selectivity of a capacitively coupled reactor.
A plasma reactor for processing a semiconductor workpiece, the reactor includes a vacuum chamber with a sidewall and a ceiling enclosing the vacuum chamber, a workpiece support pedestal within the vacuum chamber, a process gas inlet to the vacuum chamber and a vacuum pump coupled to the vacuum chamber, and an inductive antenna adjacent the vacuum chamber and an RF power source coupled to the inductive antenna. The inductive antenna includes an array of plural current conducting loops placed side-by-side and defining a surface, successive ones of the plural current conducting loops being wound so as to carry current from the RF source in opposite directions. The current conducting loops are respective conductors, at least a portion of each conductor of one loop being adjacent at least a portion of the conductor of the next loop of the array, the portions carrying current in the same direction and being sufficiently close so as to define a juncture sharing a common current path, the array having plural such junctures periodically spaced along at least one length thereof.
The spacing between the junctures corresponds to a distance traveled by an electron of a selected kinetic energy during one half cycle of the RF power source. Preferably, the selected kinetic energy is below the fluorine ionization energy band whereby excitation of electrons populates energy levels outside of the Ins fluorine ionization energy band. Alternatively, the selected kinetic energy is within the fluorine ionization energy band whereby electrons within the fluorine ionization energy band are excited to a higher energy level outside of the fluorine ionization energy band.
Preferably, the junctures are spaced uniformly. The shorter spacing of the junctures is along an inner circular path at an inner radius and a longer spacing of the junctures is along an outer circular path at an outer radius. The shorter and longer spacings correspond to distances traveled by electrons of respective higher and lower selected kinetic energies during one half cycle of the RF power source.